1. Field of the Invention
This invention relates to an automatic transmission, particularly to an automatic transmission for an automobile which is used together with a torque converter and more particularly to an automatic transmission comprising a main-transmission-mechanism such as three speed or four speed transmission mechanism and a sub-transmission-mechanism such as underdrive mechanism.
2. Description of the Prior Art
It has previously been proposed to connect a high-and-low two speed sub-transmission-mechanism including an underdrive mechanism or an overdrive mechanism consisting of one planetary gear unit with a main-transmission-mechanism of forward three speed consisting of two planetary gears so as to provide an automatic transmission of forward four speed well-known as an overdrive automatic transmission.
With this automatic transmission, one, two and three speeds as a whole are obtained by shifting the main-transmission-mechanism to one, two and three speed, respectively, while the sub-transmission-mechanism is kept at the low speed stage ( underdrive of the underdrive mechanism or direct coupling of the overdrive mechanism ) ; and further, overdrive four speed is obtained by shifting the sub-transmission-mechanism to the high speed stage ( direct coupling of the underdrive mechanism, overdrive of the overdrive mechanism ) while the main-transmission-mechanism is kept at three speed.
By the way, the structure of sub-transmission-mechanism consisting of a planetary gear ( underdrive mechanism, overdrive mechanism ) does not allow the transmission ratio to be set at not less than a certain value (0.75). In addition to this limitation, when considering problems associated with installation on vehicles, said value is subject to further limitations ( about 0.71 ).
Consequently, conventional four speed automatic transmission is restricted by the problem that the gear ratio range for the transmission stages of the sub-transmission-mechanism, namely, the gear ratio range for changing between three speed and four speed ( overdrive ) is subject to limitations, which sometimes causes problems in designing the optimum setting of gear ratio range throughout four speeds and satisfying both running performance and fuel efficiency.